The invention relates to a television camera tube. The camera tube comprises a tubular envelope of glass drawn on a mandril. The envelope has an internal wall with a conductive coating. An apertured diaphragm is supported in the envelope portion by a supporting surface extending transverse to the longitudinal axis of the envelope portion. The supporting surface is formed by a substantially stepwise change in the internal transverse dimension of the envelope portion. The conductive wall coating is interrupted at a distance from the diaphragm. The stepwise change of the internal transverse dimension of the envelope portion is made up of at least a first step and a second step, the first step forms the supporting surface for the diaphragm. The interruption in the conductive wall coating is provided on the second step.
Such a television camera tube is known from Netherlands patent application 7807758 (corresponding to U.S. Pat. No. 4,276,494). The camera tube described in this patent application comprises a spherical diaphragm which has an aperture. The spherical part of the diaphragm extends from the interruption in the wall coating into the envelope portion which is provided with the conductive coating which is electrically connected to the diaphragm. As a result of the curvature of the diaphragm, at the area of the aperture in the diaphragm the axial field strength is considerably lower than at the area of the interruption. This is necessary so as to keep the spherical aberration of the electron lens formed in the interruption small.
It has now been found that small deviations from the roundness of the spherical diaphragm, or of diaphragms having the form of a truncated cone, have an adverse influence on the electron lens. In order to avoid disturbing axial astigmatism, very high requirements have to be imposed upon the roundness of the spherical or conical diaphragm, which requirements are diffucult to meet in a metal component which has been manufactured by means of deep drawing.